As image display devices that can visually recognize three-dimensional images with no use of dedicated eyeglasses and that include a retroreflective member, an image output device, and a half mirror are widely known.
Patent Literature 1, for example, discloses a three-dimensional image projection system including a projector, a half mirror, and a retroreflective screen. In the projection system, an image projected from the projector is applied to the half mirror and reflected off the surface of the mirror. The image is then applied to the retroreflective screen, and retroreflected at the screen to the half mirror. The image again applied to the half mirror is transmitted through the half mirror, and then a high-quality three-dimensional image is displayed.
Patent Literature 2 discloses an image-forming optical system including a half mirror and a retroreflective element for forming a real mirror image of an object to be projected. In the image-forming optical system, light emitted from the object to be projected is reflected off the half mirror. The light is retroreflected at the retroreflective element to the half mirror, and transmitted through the half mirror. A real mirror image of the object to be projected is formed at a planar symmetric position with respect to the half-mirror surface. Patent Literature 2 discloses that a cube corner reflector including three adjacent mirror faces is suited to the retroreflective element, and that the three mirror faces are directed to given directions in unit retroreflective elements of the retroreflective element, with mutual angles between the three mirror faces being maintained in the unit retroreflective elements.
Here, the retroreflective member will be described.
The sheet-like retroreflective member is specifically referred to as a retroreflective sheeting.
Typical retroreflective sheeting that are known include prismatic retroreflective sheeting and beaded retroreflective sheeting.
The prismatic retroreflective sheeting shows retroreflection performances markedly higher than the retroreflection performances of the beaded retroreflective sheeting. Thus, the prismatic retroreflective sheeting is preferably used for various signs, for example, typified by road signs and construction signs.
A prismatic retroreflective member like a prismatic retroreflective sheeting has a plurality of prismatic retroreflective elements. Commonly, a plurality of prismatic retroreflective elements is formed on one face or two faces of the retroreflective member with no gap.
A typical prismatic retroreflective element includes three planes orthogonal to one another. A light beam entered to the prismatic retroreflective element is in turn reflected off the three planes, and then retroreflected to the direction of the light source.
The prismatic retroreflective elements that are known include triangular pyramid retroreflective elements and full cube corner retroreflective elements, for example.
Typical triangular pyramid retroreflective elements are shown in FIGS. 6A and 6B of Patent Literature 2. As shown in FIG. 6B of Patent Literature 2, the retroreflective elements each of which includes three rectangular equilateral triangles in the same shape and size. The three rectangular equilateral triangles are three planes orthogonal to one another. The triangles form a triangular pyramid. When the retroreflective element is viewed from the front, a regular triangle is formed. As shown in FIG. 6A of Patent Literature 2, the triangular pyramid retroreflective elements are formed (packed) with no gap in the inside of the common plane of the typical triangular pyramid retroreflective elements.
Typical full cube corner retroreflective elements are shown in FIGS. 7A and 7B of Patent Literature 2. As shown in FIG. 7B of Patent Literature 2, the retroreflective elements each of which includes three squares in the same shape and size. The three squares are three planes orthogonal to one another. The squares form the shape of the full cube corner retroreflective element. When the retroreflective elements are viewed from the front, a regular hexagon is formed. As shown in FIG. 7A of Patent Literature 2, the full cube corner retroreflective elements are formed (packed) with no gap in the inside of the common plane of the typical full cube corner retroreflective elements.
An angle formed by the light beam incident on the retroreflective element and the normal to the surface of the retroreflective member (the face to which the light beam is entered) is referred to as the entrance angle. In the case in which a light beam is entered to the retroreflective member in the direction the same as the normal to the retroreflective member, for example, the entrance angle is zero degree.
In the prismatic retroreflective element, the retroreflection performances are suddenly degraded as the entrance angle is increased. This is because an increase in the entrance angle decreases the range where the light beam can be reflected off three planes of the retroreflective elements and increases the range where the light beam can be reflected only off two or one plane. There are known prismatic retroreflective elements with improved performances that maintain retroreflection performances even though light beams are entered at a large entrance angle, i.e. with improved entrance angle characteristics.
Patent Literature 3 discloses triangular pyramid retroreflective elements having the tilt angle of the optical axis in the range of 3 to 15°.
Patent Literature 4 discloses full cube corner retroreflective elements having the tilt angle of the optical axis in the range of 0.5 to 15°.
Commonly, when an angle formed by the light beam incident on the retroreflective element and the optical axis of the retroreflective element is small, the light beam is excellently retroreflected.
Here, in an image display device equipped with a retroreflective member, an image output device, and a half mirror, a light beam (an image) outputted from the image output device is mirror-reflected off the half mirror, entered to the retroreflective member at an entrance angle of zero degree or more, and then retroreflected in the direction in which the light beam has been entered.
From FIG. 1 of Patent Literature 1 and FIG. 1B of Patent Literature 2, a light beam (an image) is entered to the retroreflective member equipped on the image display device at an entrance angle above zero degree.
[Patent Literature 1] JP-A-2001-66696
[Patent Literature 2] JP-A-2009-25776
[Patent Literature 3] JP-A-11-305018
[Patent Literature 4] WO 2010/067583 A